An effect of a lift force on the structure of attached internal waves in a continuously stratified fluid (original) (raw)
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Dynamics of Atmospheres and Oceans, 2001
Experimental investigations of fine and macroscopic structures of density and velocity disturbances generated by a towing cylinder or a vertical strip in a linearly stratified liquid are carried out in a rectangular tank. A density gradient field is visualised by different Schlieren methods (direct shadow, 'slit-knife', 'slit-thread', 'natural rainbow') characterised by a high spatial resolution. Profiles of fluid velocity are visualised by density markers-wakes past a vertically descending sugar crystal or an ascending gas bubble. In a fluid at rest, the density marker acts as a vertical linear source of internal oscillations which allows us to measure buoyancy frequency over all depth by the Schlieren instrument directly or by a conductivity probe in a particular point. Sensitive methods reveal a set of high gradient interfaces inside and outside the downstream wake besides well-known large scale elements: upstream disturbances, attached internal waves and vortices. Solitary interfaces located inside the attached internal waves field have no features on their leading and trailing edges. A thickness of interfaces is defined by an appropriate diffusion coefficient and a buoyancy frequency. High gradient interfaces bound compact vortices. Vortices moving with respect to environment emit their own systems of internal waves randomising a regular pattern of attached antisymmetric internal waves. But after a rather long time a wave recurrence occurs and a regular but symmetric structure of the longest waves (similar to the pattern of initial attached internal waves) is observed again. High gradient interfaces and lines of their intersections act as collectors of a dye coming from a compact source or from a coloured liquid volume inside the tank and separate coloured and clear areas.
Fluid Dynamics, 2013
High-resolution shadow visualization and high-frequency sonar detection are applied to separate out the density wake and a fine streaky structure in the vicinity of a vertical plate in motion in salt-stratified water. The length of the sounding acoustic wave is taken to be approximately equal to the universal microscale δ ν N = √ ν/N, where ν and N are the kinematic viscosity and the buoyance frequency. In the spectra of the vertical oscillations of the acoustic contrast some characteristic frequencies ω are separated out and used to calculate the local Stokes microscales δ ν ω = √ ν/ω in the density wake region. The scales determined from the data of independent optical and acoustic measurements are in agreement with each other.
Flow Visualization of Internal Waves and Wakes of a Streamlined Body in a Stratified Fluid
Journal of Applied Fluid Mechanics
The wake and internal waves of a moving three dimensional (3D) airfoil body in a stratified fluid has been investigated in a large stratified tank with a finite depth using movies of shadowgraphs of the flow fields. Typical Reynolds and Froude numbers of the flow varied between 10 3 and 10 4 , and 0.3 and 2 respectively. The flows are generated often by towing the body in a uniformly stratified flow, while limited cases are carried out with body stationary and the channel was in recirculating mode. For some experiments the density profile had a stepped like shape. The wake flow is often consisted of internal waves including random and coherent ones. Distortion of density fields was also observed ahead and above the body in cases where the Froude number was subcritical. Results show that as the Froude number (Fr=U/Nh, where U is the body relative velocity, N is buoyancy frequency and h is the thickness of the body) is increased, the flow undergoes from a subcritical narrow wake (for Fr<1) to an internal waves dominated flow (for Fr~1) and then to a hydraulic jump with a turbulent wake with some mixing (for Fr>1). Typical wavelength of the exited internal waves is increased with Fr, as the theory predicts. The wake of the flow for Fr>1.4 appeared to collapse and some internal waves emission from it could be observed. Usually two types of internal waves, namely random small scale and large scale, more regular waves are observed.
Laboratory, numerical, and theoretical modeling of the flow in a far wake in a stratified fluid
Izvestiya, Atmospheric and Oceanic Physics, 2006
The far-wake flow past a sphere towed in a fluid with high Reynolds and Froude numbers and with a pycnocline-form salt-density stratification is studied in a laboratory experiment based on particle image velocimetry and in numerical and theoretical modeling. In the configuration under consideration, the axis of sphere towing is located under a pycnocline. Flow parameters, the profiles of density and average velocity, and the initial field of velocity fluctuation in numerical modeling are specified from the data of the laboratory experiment. The fields of fluid velocity at different times and the time dependences of integral parameters of wake flow, such as the average velocity at the axis and the transverse width of the flow, are obtained. The results of numerical modeling are in good qualitative and quantitative agreement with the data of the laboratory experiment. The results of the laboratory experiment and numerical modeling are compared to the predictions of a quasi-linear and quasi-two-dimensional theoretical model. The time evolution of both the average velocity at the axis and the transverse width of the wake is obtained with the model and is in good agreement with the experimental data. The results of numerical modeling also show that, under the effect of velocity fluctuation in the wake, internal waves whose spatial period is equal to the characteristic period of the wake's vortex structure are excited efficiently in the pycnocline.
On the structure and dynamics of stratified wakes generated by submerged propagating objects
Journal of Operational Oceanography, 2017
The structure and intensity of the intermediate wake generated by a submerged propagating body in a stratified fluid was studied using a combination of (i) numerical simulations, (ii) field measurements, and (iii) laboratory experiments. The numerical component offered guidance for the field work performed in Monterey Bay (CA, USA) in the summer of 2015. The field work focused on subsurface thermal signatures of a submerged propagating object. Vertical temperature profiles suggested that long-term changes in thermal stratification can occur after the passage of a towed body. Horizontal temperature variability, measured by an autonomous underwater vehicle facilitated the identification of the wake using perturbation temperature variance as the key diagnostic variable. Analogous thermal signatures of stratified wakes were found in ocean observations and in modelling results. The influence of the tow ship on the wake was shown to be minimal. Laboratory experiments focused on the surface expression of stratified wakes were used to complement numerical simulations and field measurements. All three components of this project indicate that detection of the wake of a submerged object based on its thermal signatures is a viable and effective approach.
2004
A combination of ray and Fourier methods is used to describe the linear internal wavefield generated by a horizontally moving, vertically oscillating, source in a stratified fluid. Ray theory is used to approximate the wavefield in a Fourier transform domain. The ray solutions are then superimposed by inverse Fourier transform to produce the spatial solution. This is a more practical approach than calculating the ray solution directly in the spatial domain, and it is general enough to treat background flows with depth dependent shear and stratification. The theory is compared with measurements of the internal wavefield generated in tank experiments by a towed sphere in a uniformly stratified background.