Wall Jet Boundary Layer Research Articles

Sound radiated by three-dimensional discontinuities immersed in a turbulent boundary layer

Measurements of the far-field sound from two types of three-dimensional discontinuities – swept forward-facing steps and a circular embossment – immersed in a low Mach number (M<0.18) turbulent boundary layer were made. The discontinuity height in each case was smaller than the height of the approaching, undisturbed boundary layer and the Reynolds number based on the discontinuity height ranged between 8300–92,000. The swept forward-facing steps were immersed in a wall-jet boundary layer, whereas the embossment was placed in a conventional boundary layer. For the embossment, measurements of surface pressure fluctuations were also performed beneath the separated flow on the vertical and the downstream surfaces. The wall-jet measurements show that the far-field sound from swept forward-facing step scales on the step-normal component of the boundary-layer edge velocity. The far-field sound from the circular embossment is qualitatively similar to that from a forward-facing step of the same height, a fact which is also reflected in the surface pressure fluctuation measurements. The step-normal velocity scaling obtained for the swept forward-facing steps was combined with an existing prediction formula for the sound radiated by a forward-facing step to yield a straight-forward prediction formula for the embossment noise. The prediction formula shows good agreement with the measured sound spectra if it is assumed that the dominant source for the embossment is the separation/reattachment region concentrated in the forward arc of the embossment. This assumption is supported by oil flow visualization which shows that the separated flow region is concentrated between approximately ±45°on either side of the spanwise center.

Sound radiated by turbulent flow over unrounded and rounded forward-facing steps

This study presents measurements of the far-field noise radiated by forward-facing steps with and without rounded corners. A noise prediction formula, obtained by combining theory and measurements, to predict noise radiated by an unrounded forward step is also presented. The prediction methodology has also been extended to include the effects of rounding the step corner. The height of the step in measurements ranged between 78% to 84% of the incoming, turbulent wall-jet boundary layer. The Reynolds number based on step height was between 8300 and 16,800. Six roundings of the step corner were considered with radii equal to 0%, 6.25%, 12.5%, 25%, 50%, and 100% of the step height. It was found that rounding the forward step corner reduces the noise without affecting its directivity and scaling significantly. A slight rounding of the corner only affects the noise at high frequencies, however, a further increase in rounding leads to a broadband noise reduction. The prediction formula for the unrounded forward step yields reasonably good agreement with both past and present measurements. The predictions also suggest that the noise generated by a forward step is independent of the incoming boundary-layer thickness and scales with the step height. The predictions for rounded forward steps show that the noise attenuation due to corner rounding can be reasonably approximated if it is assumed that the unsteady surface loading in the vicinity of rounded steps scales with the reattachment length associated with each rounding.

Hydrodynamics around a shroud tube assembly of a fast breeder reactor

Control plug assembly of a liquid sodium cooled fast breeder reactor is partially submerged inside the liquid sodium coolant pool, free surface of which is blanketed with cover gas, mainly Argon to provide a non-reactive environment. It receives high velocity coolant flow from the fuel subassembly which travels through annular gap inside Control Safety Rod Driving Mechanism (CSRDM) assembly leading to swelling of free surface and gas entrainment inside the coolant pool. Entrained gas bubbles travel through different submerged components of the reactor and induces numerous complications including reactor power fluctuation, cavitation in inlet pumps and heat transfer inhibition in heat exchangers. Study of hydrodynamics around the shroud tube is crucial to examine cover gas entrainment inside the pool. The present study reports an experimental study on a 1:9 scale down model of CSRDM shroud tube assembly based on Froude number similarity. Eccentricity of the control rod inside the shroud tube is also an important parameter as temperature gradient along the CSRDM assembly leads to longitudinal thermal stress causing displacement of control rod from its center position in the shroud tube. Flow behavior around shroud tube, hump formation at the axial exit of shroud tube, air entrainment inside coolant pool have been reported for various parameters viz. inlet flow rate (qi), free surface height (hf) and eccentricity of central rod (e). Dye visualization, Schlieren visualization, Bulk visualization have been carried out and digital image processing techniques have been adopted for quantification of hump parameters and entrained air fraction inside the coolant pool.The axial flow from the exit of the shroud tube impinges on the free surface like a wall jet. At low Froude number, the impinging jet flow travels radially outward on the interface like a wall jet boundary layer. The shear of the free surface flow drags the jet like flow from the pores upward leading to mixing between the vortices inside the shear layer and the free vortices from the pores of the shroud tube. At high Froude number, the impinging jet like flow penetrates through the interface leading to hump formation. The downward motion of the hump reduces the radial free surface flow and blankets the radial flow from the pores of the shroud tube. The eccentricity of the central rod leads to asymmetric axial flow from the shroud tube and thus the resulting hump becomes asymmetric due to impingement of asymmetric jet. Therefore, the impingement angle of the downward hump motion becomes different at both sides of the shroud tube with asymmetric flow field and bubble formation process inside the liquid pool. However, the synchronization between the hump unsteadiness and the surface wave motion leads to mixing of air bubbles inside the liquid pool. Average entrained air fraction increases with increase in inlet flow rate and decreases with increase in free surface height.

Measurements of roughness noise

Experiments have been performed on the roughness noise produced by a two-dimensional turbulent wall jet boundary layer flowing over short fetches of sandpaper roughness. A range of rough surface sizes were studied from hydrodynamically smooth through fully rough. Velocity measurements were made to document the form of the wall jet boundary layer and the influence of the roughness upon it. Acoustic measurements showed background noise levels to be very low so that the sound produced by the rough surfaces could be clearly detected with signal to noise ratios as large as 20 dB. Even hydrodynamically smooth roughness was found to produce noise, conclusively indicating the presence of scattering as a source mechanism. Variations of the roughness noise spectra with flow speed and roughness size are found to be inconsistent with any simple parameter scaling. Boundary layer wall pressure fluctuation measurements made within the roughness fetches reveal a spectral form quite different than the roughness noise, and fluctuation levels some 50–70 dB higher. Despite these differences the wall pressure and roughness noise are found to be very simply related, at least at lower frequencies (<6 kHz). The roughness noise spectrum varies closely as the product of the wall pressure spectrum, the frequency squared, and the mean-square roughness height. This is the scaling predicted by scattering theory and implies a major simplification to the problem of roughness noise prediction for stochastic surfaces.

A simple estimation of the growth rate and equilibrium size of bedforms created by a turbulent wall jet

A simple procedure for predicting the time evolution and equilibrium size of the two dimensional bedform created by a turbulent wall jet is presented. The model applies to a planar jet discharging flush with and parallel to an initially horizontal bed of cohesionless sediment. Additionally, the water is assumed to be deep relative to the initial jet thickness. The model is developed using existing free- and wall-jet boundary layer relations and existing sediment incipient motion and bedload transport equations. The model is validated against live-bed experiments from previous studies and from a new set of laboratory experiments. The agreement between the predictions for and observations of equilibrium size is quite good. The agreement regarding the time evolution is also good. These results suggest that several key characteristics of jet-induced bedforms are captured with a limited physical approach.

The Steady Horizontal Flow of a Wall Jet Into a Large-Width Cavity

This paper treats the steady flow of a wall jet into a large-width cavity for which the primary axis is normal to the direction of the jet inflow. Numerical solutions of the two-dimensional Navier-Stokes equations are computed for inlet Reynolds numbers of 10 to 50 and tank width to inlet height ratios of 16 to 128. The length and velocity scales of the wall jet boundary layer exhibit close agreement with the classic wall jet similarity solution and published experimental data but the width of the region for which the comparison proves to be favorable has a limited extent. This departure from a self-similar evolution of the wall jet is shown to result from the finite domain width and its influence on the large recirculation cell located immediately above the wall jet boundary layer.

Unsteady separation in a boundary layer produced by an impinging jet

A vortex-induced unsteady separation was investigated experimentally in the laminar boundary layer produced by an axisymmetric jet impinging normally onto a flat plate. By forcing the air jet, primary ring vortices were periodically generated in the jet shear layer. Phase-locked flow visualization showed that the wall-jet boundary layer separated periodically and evolved into a secondary vortex counter rotating with respect to the primary vortex. The unsteady separation is induced by the primary vortex and moves downstream in the radial mean-flow direction. Phase-averaged hot-wire measurements using a parallel-wire sensor in the vicinity of the unsteady separation provided data for locating the onset of separation in space and time. The data revealed that the unsteady separation originated from a local shear layer which was initiated by the unsteady adverse pressure gradient produced by the primary vortex.

Heat Transfer to Plane Turbulent Wall Jets

The heat-transfer characteristics of two-dimensional, incompressible, turbulent wall jets are discussed. An analytical prediction is made for the local Stanton number and data are presented for a step wall temperature distribution. The method for extending these data to arbitrary heating conditions is shown. Temperature surveys in the wall jet boundary layer are also presented.