Juha Kivekäs - Academia.edu (original) (raw)

Papers by Juha Kivekäs

ISBS - Conference Proceedings Archive, 2000

Naked 2/3 scale model without and with reference skis, four different pairs of skis and two diffe... more Naked 2/3 scale model without and with reference skis, four different pairs of skis and two different suits were evaluated in the 2 * 2 m 2 low speed wind tunnel. The tests were carried out using 30-35 ms-1 flow speed equaling Reynolds number 2.4 * 10 6. The positions of angle of attack ranged from-8 degree to +12 degree. Reference position 0-degree equals incidence of 30 degree of plane defined by skis with zero flow speed. The aerodynamic characteristics were drag, lift and pitching moment, which were transformed into dimensionless coefficients. In general, it can be stated that 60 % of the aerodynamic forces were acting on the body of the model and 40 % on the skis. If the ski area increased by 1 % the aerodynamic forces increased 0.4 %. The relative increments in the length of the jump in a K 120 jumping hill with the skis A, B and C were 5.5 %, 6.2 % and 5.1 %, respectively. The effects of variation on air permeability of the suits were on the drag 7-12 % and lift 12-25 %.

Sports Biomechanics, Jan 28, 2019

In ski jumping low body weight development resulted in some serious underweight problems and ther... more In ski jumping low body weight development resulted in some serious underweight problems and therefore International Ski Federation (FIS) decided to solve the problem by relating maximum ski length to Body Mass Index (BMI). The present study examined the current relationship between body weight (BMI), ski length and performance (jumping distance) in ski jumping. The computer simulation showed that BMI regulation adopted by FIS for ski jumping in 2004 has reduced the effect of body weight on jumping distance, but despite use of shorter skis it is still advantageous to be light within certain limits.

Sports Engineering, Feb 8, 2019

A single isolated ski was suspended from a six-component wind tunnel balance and three angles, th... more A single isolated ski was suspended from a six-component wind tunnel balance and three angles, the angle of attack, the yaw angle and the edge angle were adjustable during the test. Increasing yaw angle from 0 to 15° increased the lift coefficient C L from 0.42 to 0.90 at edge angle 0° and from 0.70 to 0.87 at edge angle 10°, respectively. Increasing yaw angle also increased the sensitivity of the ski to changes in edge angle, i.e., increasing the edge angle (20°-45°) decreased the C L and the ratio C 2 L ∕C D with large yaw angles. However, to maximize the lift-to-drag ratio with a typical angle of attack of 30° in ski jumping, it may be reasonable to have an edge angle of 5°-10° on skis as the ratio C 2 L ∕C D increased from 1.24 to 1.35 when edge angle increased from 0° to 10°.

Journal of Applied Biomechanics, Nov 1, 2011

The effect of skis on the force-time characteristics of the simulated ski jumping takeoff was exa... more The effect of skis on the force-time characteristics of the simulated ski jumping takeoff was examined in a wind tunnel. Takeoff forces were recorded with a force plate installed under the tunnel floor. Signals from the front and rear parts of the force plate were collected separately to examine the anteroposterior balance of the jumpers during the takeoff. Two ski jumpers performed simulated takeoffs, first without skis in nonwind conditions and in various wind conditions. Thereafter, the same experiments were repeated with skis. The jumpers were able to perform very natural takeoff actions (similar to the actual takeoff) with skis in wind tunnel. According to the subjective feeling of the jumpers, the simulated ski jumping takeoff with skis was even easier to perform than the earlier trials without skis. Skis did not much influence the force levels produced during the takeoff but they still changed the force distribution under the feet. Contribution of the forces produced under the rear part of the feet was emphasized probably because the strong dorsiflexion is needed for lifting the skis to the proper flight position. The results presented in this experiment emphasize that research on ski jumping takeoff can be advanced by using wind tunnels.

Sports Biomechanics, Sep 1, 2012

The present study examined the effect of wind on jumping distance in ski jumping by means of comp... more The present study examined the effect of wind on jumping distance in ski jumping by means of computer simulation and compared the results with the wind compensation factors adopted recently in ski jumping competitions by FIS. The final result seems to be very much dependent on the jump length of the generic jump used as a basis for the computer simulations and therefore the differences in jumpers' performance level make the solution of the compensation system very difficult.

Journal of Biomechanics, Apr 1, 2001

The effect of aerodynamic forces on the force-time characteristics of the simulated ski jumping t... more The effect of aerodynamic forces on the force-time characteristics of the simulated ski jumping takeoff was examined in a wind tunnel. Vertical and horizontal ground reaction forces were recorded with a force plate installed under the wind tunnel floor. The jumpers performed takeoffs in non-wind conditions and in various wind conditions (21-33 m s À1). EMGs of the important takeoff muscles were recorded from one jumper. The dramatic decrease in takeoff time found in all jumpers can be considered as the result of the influence of aerodynamic lift. The loss in impulse due to the shorter force production time with the same takeoff force is compensated with the increase in lift force, resulting in a higher vertical velocity (V v) than is expected from the conventional calculation of V v from the force impulse. The wind conditions emphasized the explosiveness of the ski jumping takeoff. The aerodynamic lift and drag forces which characterize the aerodynamic quality of the initial takeoff position (static in-run position) varied widely even between the examined elite ski jumpers. According to the computer simulation these differences can decisively affect jumping distance. The proper utilization of the prevailing aerodynamic forces before and during takeoff is a very important prerequisite for achieving a good flight position.

INTRODUCTION Wind tunnels have traditionally been used to study the aerodynamic properties of the... more INTRODUCTION Wind tunnels have traditionally been used to study the aerodynamic properties of the objects whose perform ance is highly dependent on the aerodynamic forces gener ated by air flow. Ski jumping is certainly one of those areas where the wind tunnel research provides a great pos sibility to improve the understanding of successful performa nce [1]. This abstract gives examples of experiments wh ich have been used to study the effects of ski jumping suit design on jumping performance (i.e. jumping distanc e). This was done by applying wind tunnel results to a computer simulation.

Naked 2/3 scale model without and with reference skis, four different pairs of skis and two diffe... more Naked 2/3 scale model without and with reference skis, four different pairs of skis and two different suits were evaluated in the 2 * 2 m2 low speed wind tunnel. The tests were car-ried out using 30–35 ms–1 flow speed equaling Reynolds number 2.4 * 106. The positions of angle of attack ranged from -8 degree to +12 degree. Reference position 0-degree equals incidence of 30 degree of plane defined by skis with zero flow speed. The aero-dynamic characteristics were drag, lift and pitching moment, which were transformed into dimensionless coefficients. In general, it can be stated that 60 % of the aerodynamic forces were acting on the body of the model and 40 % on the skis. If the ski area in-creased by 1 % the aerodynamic forces increased 0.4 %. The relative increments in the length of the jump in a K 120 jumping hill with the skis A, B and C were 5.5 %, 6.2 % and 5.1 %, respectively. The effects of variation on air permeability of the suits were on the drag 7–12 % and lift 12–25 %.

Sports Engineering

A single isolated ski was suspended from a six-component wind tunnel balance and three angles, th... more A single isolated ski was suspended from a six-component wind tunnel balance and three angles, the angle of attack, the yaw angle and the edge angle were adjustable during the test. Increasing yaw angle from 0 to 15° increased the lift coefficient C L from 0.42 to 0.90 at edge angle 0° and from 0.70 to 0.87 at edge angle 10°, respectively. Increasing yaw angle also increased the sensitivity of the ski to changes in edge angle, i.e., increasing the edge angle (20°-45°) decreased the C L and the ratio C 2 L ∕C D with large yaw angles. However, to maximize the lift-to-drag ratio with a typical angle of attack of 30° in ski jumping, it may be reasonable to have an edge angle of 5°-10° on skis as the ratio C 2 L ∕C D increased from 1.24 to 1.35 when edge angle increased from 0° to 10°.

Journal of applied biomechanics, 2011

The effect of skis on the force-time characteristics of the simulated ski jumping takeoff was exa... more The effect of skis on the force-time characteristics of the simulated ski jumping takeoff was examined in a wind tunnel. Takeoff forces were recorded with a force plate installed under the tunnel floor. Signals from the front and rear parts of the force plate were collected separately to examine the anteroposterior balance of the jumpers during the takeoff. Two ski jumpers performed simulated takeoffs, first without skis in nonwind conditions and in various wind conditions. Thereafter, the same experiments were repeated with skis. The jumpers were able to perform very natural takeoff actions (similar to the actual takeoff) with skis in wind tunnel. According to the subjective feeling of the jumpers, the simulated ski jumping takeoff with skis was even easier to perform than the earlier trials without skis. Skis did not much influence the force levels produced during the takeoff but they still changed the force distribution under the feet. Contribution of the forces produced under th...

INTRODUCTION: With the emergence of the V-style jumping technique, ski jump length was dramatical... more INTRODUCTION: With the emergence of the V-style jumping technique, ski jump length was dramatically increased (Mahnke and Hochmuth, 1990). In order to counteract, two factors affecting on the aerodynamic efficiency were started to be regulated; the length of the jump- ing skis and the ski suit permeability. The first rule allowed the ski length to be one's body height plus

Sports Biomechanics, 2012

The special wind compensation system recently adopted by Fédération Internationale de Ski (FIS; I... more The special wind compensation system recently adopted by Fédération Internationale de Ski (FIS; International Ski Federation) to consider the effects of changing wind conditions has caused some controversy. Here, the effect of wind on jumping distance in ski jumping was studied by means of computer simulation and compared with the wind compensation factors used by FIS during the World Cup season 2009/2010. The results showed clearly that the effect of increasing head/tail wind on jumping distance is not linear: +17.4 m/-29.1 m, respectively, for a wind speed of 3 m/s. The linear formula used in the trial period of the wind compensation system was found to be appropriate only for a limited range of jumping distances as the gradient of the landing slope slows down the rate of distance change in long jumps.

Journal of Biomechanics, 2001

The effect of aerodynamic forces on the force-time characteristics of the simulated ski jumping t... more The effect of aerodynamic forces on the force-time characteristics of the simulated ski jumping take-off was examined in a wind tunnel. Vertical and horizontal ground reaction forces were recorded with a force plate installed under the wind tunnel floor. The jumpers performed take-offs in non-wind conditions and in various wind conditions (21-33 m s(-1)). EMGs of the important take-off muscles were recorded from one jumper. The dramatic decrease in take-off time found in all jumpers can be considered as the result of the influence of aerodynamic lift. The loss in impulse due to the shorter force production time with the same take-off force is compensated with the increase in lift force, resulting in a higher vertical velocity (V(v)) than is expected from the conventional calculation of V(v) from the force impulse. The wind conditions emphasized the explosiveness of the ski jumping take-off. The aerodynamic lift and drag forces which characterize the aerodynamic quality of the initial take-off position (static in-run position) varied widely even between the examined elite ski jumpers. According to the computer simulation these differences can decisively affect jumping distance. The proper utilization of the prevailing aerodynamic forces before and during take-off is a very important prerequisite for achieving a good flight position.

Journal of Biomechanics, 2001

The effect of aerodynamic forces on the force-time characteristics of the simulated ski jumping t... more The effect of aerodynamic forces on the force-time characteristics of the simulated ski jumping take-off was examined in a wind tunnel. Vertical and horizontal ground reaction forces were recorded with a force plate installed under the wind tunnel floor. The jumpers performed take-offs in non-wind conditions and in various wind conditions (21-33 m s(-1)). EMGs of the important take-off muscles were recorded from one jumper. The dramatic decrease in take-off time found in all jumpers can be considered as the result of the influence of aerodynamic lift. The loss in impulse due to the shorter force production time with the same take-off force is compensated with the increase in lift force, resulting in a higher vertical velocity (V(v)) than is expected from the conventional calculation of V(v) from the force impulse. The wind conditions emphasized the explosiveness of the ski jumping take-off. The aerodynamic lift and drag forces which characterize the aerodynamic quality of the initial take-off position (static in-run position) varied widely even between the examined elite ski jumpers. According to the computer simulation these differences can decisively affect jumping distance. The proper utilization of the prevailing aerodynamic forces before and during take-off is a very important prerequisite for achieving a good flight position.

ISBS - Conference Proceedings Archive, 2000

Naked 2/3 scale model without and with reference skis, four different pairs of skis and two diffe... more Naked 2/3 scale model without and with reference skis, four different pairs of skis and two different suits were evaluated in the 2 * 2 m 2 low speed wind tunnel. The tests were carried out using 30-35 ms-1 flow speed equaling Reynolds number 2.4 * 10 6. The positions of angle of attack ranged from-8 degree to +12 degree. Reference position 0-degree equals incidence of 30 degree of plane defined by skis with zero flow speed. The aerodynamic characteristics were drag, lift and pitching moment, which were transformed into dimensionless coefficients. In general, it can be stated that 60 % of the aerodynamic forces were acting on the body of the model and 40 % on the skis. If the ski area increased by 1 % the aerodynamic forces increased 0.4 %. The relative increments in the length of the jump in a K 120 jumping hill with the skis A, B and C were 5.5 %, 6.2 % and 5.1 %, respectively. The effects of variation on air permeability of the suits were on the drag 7-12 % and lift 12-25 %.

Sports Biomechanics, Jan 28, 2019

In ski jumping low body weight development resulted in some serious underweight problems and ther... more In ski jumping low body weight development resulted in some serious underweight problems and therefore International Ski Federation (FIS) decided to solve the problem by relating maximum ski length to Body Mass Index (BMI). The present study examined the current relationship between body weight (BMI), ski length and performance (jumping distance) in ski jumping. The computer simulation showed that BMI regulation adopted by FIS for ski jumping in 2004 has reduced the effect of body weight on jumping distance, but despite use of shorter skis it is still advantageous to be light within certain limits.

Sports Engineering, Feb 8, 2019

A single isolated ski was suspended from a six-component wind tunnel balance and three angles, th... more A single isolated ski was suspended from a six-component wind tunnel balance and three angles, the angle of attack, the yaw angle and the edge angle were adjustable during the test. Increasing yaw angle from 0 to 15° increased the lift coefficient C L from 0.42 to 0.90 at edge angle 0° and from 0.70 to 0.87 at edge angle 10°, respectively. Increasing yaw angle also increased the sensitivity of the ski to changes in edge angle, i.e., increasing the edge angle (20°-45°) decreased the C L and the ratio C 2 L ∕C D with large yaw angles. However, to maximize the lift-to-drag ratio with a typical angle of attack of 30° in ski jumping, it may be reasonable to have an edge angle of 5°-10° on skis as the ratio C 2 L ∕C D increased from 1.24 to 1.35 when edge angle increased from 0° to 10°.

Journal of Applied Biomechanics, Nov 1, 2011

The effect of skis on the force-time characteristics of the simulated ski jumping takeoff was exa... more The effect of skis on the force-time characteristics of the simulated ski jumping takeoff was examined in a wind tunnel. Takeoff forces were recorded with a force plate installed under the tunnel floor. Signals from the front and rear parts of the force plate were collected separately to examine the anteroposterior balance of the jumpers during the takeoff. Two ski jumpers performed simulated takeoffs, first without skis in nonwind conditions and in various wind conditions. Thereafter, the same experiments were repeated with skis. The jumpers were able to perform very natural takeoff actions (similar to the actual takeoff) with skis in wind tunnel. According to the subjective feeling of the jumpers, the simulated ski jumping takeoff with skis was even easier to perform than the earlier trials without skis. Skis did not much influence the force levels produced during the takeoff but they still changed the force distribution under the feet. Contribution of the forces produced under the rear part of the feet was emphasized probably because the strong dorsiflexion is needed for lifting the skis to the proper flight position. The results presented in this experiment emphasize that research on ski jumping takeoff can be advanced by using wind tunnels.

Sports Biomechanics, Sep 1, 2012

The present study examined the effect of wind on jumping distance in ski jumping by means of comp... more The present study examined the effect of wind on jumping distance in ski jumping by means of computer simulation and compared the results with the wind compensation factors adopted recently in ski jumping competitions by FIS. The final result seems to be very much dependent on the jump length of the generic jump used as a basis for the computer simulations and therefore the differences in jumpers' performance level make the solution of the compensation system very difficult.

Journal of Biomechanics, Apr 1, 2001

The effect of aerodynamic forces on the force-time characteristics of the simulated ski jumping t... more The effect of aerodynamic forces on the force-time characteristics of the simulated ski jumping takeoff was examined in a wind tunnel. Vertical and horizontal ground reaction forces were recorded with a force plate installed under the wind tunnel floor. The jumpers performed takeoffs in non-wind conditions and in various wind conditions (21-33 m s À1). EMGs of the important takeoff muscles were recorded from one jumper. The dramatic decrease in takeoff time found in all jumpers can be considered as the result of the influence of aerodynamic lift. The loss in impulse due to the shorter force production time with the same takeoff force is compensated with the increase in lift force, resulting in a higher vertical velocity (V v) than is expected from the conventional calculation of V v from the force impulse. The wind conditions emphasized the explosiveness of the ski jumping takeoff. The aerodynamic lift and drag forces which characterize the aerodynamic quality of the initial takeoff position (static in-run position) varied widely even between the examined elite ski jumpers. According to the computer simulation these differences can decisively affect jumping distance. The proper utilization of the prevailing aerodynamic forces before and during takeoff is a very important prerequisite for achieving a good flight position.

INTRODUCTION Wind tunnels have traditionally been used to study the aerodynamic properties of the... more INTRODUCTION Wind tunnels have traditionally been used to study the aerodynamic properties of the objects whose perform ance is highly dependent on the aerodynamic forces gener ated by air flow. Ski jumping is certainly one of those areas where the wind tunnel research provides a great pos sibility to improve the understanding of successful performa nce [1]. This abstract gives examples of experiments wh ich have been used to study the effects of ski jumping suit design on jumping performance (i.e. jumping distanc e). This was done by applying wind tunnel results to a computer simulation.

Naked 2/3 scale model without and with reference skis, four different pairs of skis and two diffe... more Naked 2/3 scale model without and with reference skis, four different pairs of skis and two different suits were evaluated in the 2 * 2 m2 low speed wind tunnel. The tests were car-ried out using 30–35 ms–1 flow speed equaling Reynolds number 2.4 * 106. The positions of angle of attack ranged from -8 degree to +12 degree. Reference position 0-degree equals incidence of 30 degree of plane defined by skis with zero flow speed. The aero-dynamic characteristics were drag, lift and pitching moment, which were transformed into dimensionless coefficients. In general, it can be stated that 60 % of the aerodynamic forces were acting on the body of the model and 40 % on the skis. If the ski area in-creased by 1 % the aerodynamic forces increased 0.4 %. The relative increments in the length of the jump in a K 120 jumping hill with the skis A, B and C were 5.5 %, 6.2 % and 5.1 %, respectively. The effects of variation on air permeability of the suits were on the drag 7–12 % and lift 12–25 %.

Sports Engineering

A single isolated ski was suspended from a six-component wind tunnel balance and three angles, th... more A single isolated ski was suspended from a six-component wind tunnel balance and three angles, the angle of attack, the yaw angle and the edge angle were adjustable during the test. Increasing yaw angle from 0 to 15° increased the lift coefficient C L from 0.42 to 0.90 at edge angle 0° and from 0.70 to 0.87 at edge angle 10°, respectively. Increasing yaw angle also increased the sensitivity of the ski to changes in edge angle, i.e., increasing the edge angle (20°-45°) decreased the C L and the ratio C 2 L ∕C D with large yaw angles. However, to maximize the lift-to-drag ratio with a typical angle of attack of 30° in ski jumping, it may be reasonable to have an edge angle of 5°-10° on skis as the ratio C 2 L ∕C D increased from 1.24 to 1.35 when edge angle increased from 0° to 10°.

Journal of applied biomechanics, 2011

The effect of skis on the force-time characteristics of the simulated ski jumping takeoff was exa... more The effect of skis on the force-time characteristics of the simulated ski jumping takeoff was examined in a wind tunnel. Takeoff forces were recorded with a force plate installed under the tunnel floor. Signals from the front and rear parts of the force plate were collected separately to examine the anteroposterior balance of the jumpers during the takeoff. Two ski jumpers performed simulated takeoffs, first without skis in nonwind conditions and in various wind conditions. Thereafter, the same experiments were repeated with skis. The jumpers were able to perform very natural takeoff actions (similar to the actual takeoff) with skis in wind tunnel. According to the subjective feeling of the jumpers, the simulated ski jumping takeoff with skis was even easier to perform than the earlier trials without skis. Skis did not much influence the force levels produced during the takeoff but they still changed the force distribution under the feet. Contribution of the forces produced under th...

INTRODUCTION: With the emergence of the V-style jumping technique, ski jump length was dramatical... more INTRODUCTION: With the emergence of the V-style jumping technique, ski jump length was dramatically increased (Mahnke and Hochmuth, 1990). In order to counteract, two factors affecting on the aerodynamic efficiency were started to be regulated; the length of the jump- ing skis and the ski suit permeability. The first rule allowed the ski length to be one's body height plus

Sports Biomechanics, 2012

The special wind compensation system recently adopted by Fédération Internationale de Ski (FIS; I... more The special wind compensation system recently adopted by Fédération Internationale de Ski (FIS; International Ski Federation) to consider the effects of changing wind conditions has caused some controversy. Here, the effect of wind on jumping distance in ski jumping was studied by means of computer simulation and compared with the wind compensation factors used by FIS during the World Cup season 2009/2010. The results showed clearly that the effect of increasing head/tail wind on jumping distance is not linear: +17.4 m/-29.1 m, respectively, for a wind speed of 3 m/s. The linear formula used in the trial period of the wind compensation system was found to be appropriate only for a limited range of jumping distances as the gradient of the landing slope slows down the rate of distance change in long jumps.

Journal of Biomechanics, 2001

The effect of aerodynamic forces on the force-time characteristics of the simulated ski jumping t... more The effect of aerodynamic forces on the force-time characteristics of the simulated ski jumping take-off was examined in a wind tunnel. Vertical and horizontal ground reaction forces were recorded with a force plate installed under the wind tunnel floor. The jumpers performed take-offs in non-wind conditions and in various wind conditions (21-33 m s(-1)). EMGs of the important take-off muscles were recorded from one jumper. The dramatic decrease in take-off time found in all jumpers can be considered as the result of the influence of aerodynamic lift. The loss in impulse due to the shorter force production time with the same take-off force is compensated with the increase in lift force, resulting in a higher vertical velocity (V(v)) than is expected from the conventional calculation of V(v) from the force impulse. The wind conditions emphasized the explosiveness of the ski jumping take-off. The aerodynamic lift and drag forces which characterize the aerodynamic quality of the initial take-off position (static in-run position) varied widely even between the examined elite ski jumpers. According to the computer simulation these differences can decisively affect jumping distance. The proper utilization of the prevailing aerodynamic forces before and during take-off is a very important prerequisite for achieving a good flight position.

Journal of Biomechanics, 2001

The effect of aerodynamic forces on the force-time characteristics of the simulated ski jumping t... more The effect of aerodynamic forces on the force-time characteristics of the simulated ski jumping take-off was examined in a wind tunnel. Vertical and horizontal ground reaction forces were recorded with a force plate installed under the wind tunnel floor. The jumpers performed take-offs in non-wind conditions and in various wind conditions (21-33 m s(-1)). EMGs of the important take-off muscles were recorded from one jumper. The dramatic decrease in take-off time found in all jumpers can be considered as the result of the influence of aerodynamic lift. The loss in impulse due to the shorter force production time with the same take-off force is compensated with the increase in lift force, resulting in a higher vertical velocity (V(v)) than is expected from the conventional calculation of V(v) from the force impulse. The wind conditions emphasized the explosiveness of the ski jumping take-off. The aerodynamic lift and drag forces which characterize the aerodynamic quality of the initial take-off position (static in-run position) varied widely even between the examined elite ski jumpers. According to the computer simulation these differences can decisively affect jumping distance. The proper utilization of the prevailing aerodynamic forces before and during take-off is a very important prerequisite for achieving a good flight position.