Turbulent Shear Stress Levels Research Articles

Unsteady turbulence in expansion waves in rivers and estuaries: an experimental study

A sudden decrease in water depth, called a negative surge or expansion wave, is characterised by a gentle change in free-surface elevation. Some geophysical applications include the ebb tide flow in macro-tidal estuaries, the rundown of swash waters and the retreating waters after maximum tsunami runup in a river channel. The upstream propagation of expansion waves against an initially steady flow was investigated in laboratory under controlled flow conditions including detailed free-surface velocity and Reynolds stress measurements. Both non-intrusive free-surface measurements and intrusive velocity measurements were conducted for relatively large Reynolds numbers with two types of bed roughness. The data showed that the propagation of expansion waves appeared to be a relatively smooth lowering to the water surface. The wave leading edge celerity data showed a characteristic trend, with a rapid acceleration immediately following the surge generation, followed by a deceleration of the leading edge surge towards an asymptotical value: $$(\mathrm{U}+\mathrm{V}_\mathrm{o})/(\mathrm{g}\times \mathrm{d}_\mathrm{o})^{1/2}=1$$ for both smooth and rough bed experiments. The results indicated that the bed roughness had little to no effect, within the experimental flow conditions. Relatively large fluctuations in free-surface elevation, velocity and turbulent shear stress were recorded beneath the leading edge of the negative surge for all flow conditions. The instantaneous turbulent shear stress levels were significantly larger than the critical shear stress for sediment erosion. The present results implied a substantial bed erosion during an expansion wave motion.

Average Passage Flow Field and Deterministic Stresses in the Tip and Hub Regions of a Multistage Turbomachine

This paper continues our effort to study the dynamics of deterministic stresses in a multistage turbomachine using experimental data. Here we focus on the tip and hub regions and compare them to midspan data obtained in previous studies. The analysis is based on data obtained in particle image velocimetry (PIV) measurements performed in the second stage of a two-stage turbomachine. A complete data set is obtained using blades and fluid with matched optical index of refraction. Previous measurements at midspan have shown that at midspan and close to design conditions, the deterministic kinetic energy is smaller than the turbulent kinetic energy. The primary contributor to the deterministic stresses at midspan is the interaction of a blade with the upstream wakes. Conversely, we find that the tip vortex is the dominant source of phase-dependent unsteadiness and deterministic stresses in the tip region. Along the trajectory of the tip vortex, the deterministic kinetic energy levels are more than one order of magnitude higher than the levels measured in the hub and midspan, and are of the same order of magnitude as the turbulent kinetic energy levels. Reasons for this trend are explained using a sample distribution of phase-averaged flow variables. Outside of the region affected by tip-vortex transport, within the rotor-stator gap and within the stator passages, the turbulent kinetic energy is still 3–4 times higher than the deterministic kinetic energy. The deterministic and turbulent shear stress levels are comparable in all spanwise locations, except for the wakes of the stator blades, where the turbulent stresses are higher. However, along the direction of tip-vortex transport, the deterministic shear stresses are about an order of magnitude higher than the turbulent shear stresses. The decay rates of deterministic kinetic energy in the hub and midspan regions are comparable to each other, whereas at the tip the decay rate is higher. The decay rates of turbulent kinetic energy are much smaller than those of the deterministic kinetic energy. The paper also examines terms in the deterministic kinetic energy transport equation. The data indicate that “deterministic production” and a new term, here called “dissipation due to turbulence,” are the dominant source/sink terms. Regions with alternating signs of deterministic production indicate that the energy transfer between the phase-averaged and average-passage flow fields can occur in both directions. The divergence of the pressure-velocity correlation, obtained from a balance of all the other terms, is dominant and appears to be much larger than the deterministic production (source/sink) term. This trend indicates that there are substantial deterministic pressure fluctuations in the flow field, especially within the rotor-stator gap and within the stator passage.

Separated Flow Transition Under Simulated Low-Pressure Turbine Airfoil Conditions—Part 1: Mean Flow and Turbulence Statistics

Boundary layer separation, transition and reattachment have been studied experimentally under low-pressure turbine airfoil conditions. Cases with Reynolds numbers (Re) ranging from 25,000 to 300,000 (based on suction surface length and exit velocity) have been considered at low (0.5%) and high (9% inlet) free-stream turbulence levels. Mean and fluctuating velocity and intermittency profiles are presented for streamwise locations all along the airfoil, and turbulent shear stress profiles are provided for the downstream region where separation and transition occur. Higher Re or free-stream turbulence level moves transition upstream. Transition is initiated in the shear layer over the separation bubble and leads to rapid boundary layer reattachment. At the lowest Re, transition did not occur before the trailing edge, and the boundary layer did not reattach. Turbulent shear stress levels can remain low in spite of high free-stream turbulence and high fluctuating streamwise velocity in the shear layer. The beginning of a significant rise in the turbulent shear stress signals the beginning of transition. A slight rise in the turbulent shear stress near the trailing edge was noted even in those cases which did not undergo transition or reattachment. The present results provide detailed documentation of the boundary layer and extend the existing database to lower Re. The present results also serve as a baseline for an investigation of turbulence spectra in Part 2 of the present paper, and for ongoing work involving transition and separation control.

Experimental Analysis of the Turbulent Shear Stresses for Distorted Supersonic Boundary Layers

An experimental analysis of the turbulent shear stresses for a supersonic boundary layer distorted by streamline curvature-induced pressure gradients was performed using laser Doppler velocimetry. Four pressure-gradient  ows were examined: a nominally zero-pressure-gradient case (M = 2.8, Reu = 1.1 3 10, b = 0.02); a favorable-pressure gradient (M = 2.9, Reu = 1.5 3 10, b = 20.5); an adverse-pressure gradient (M = 2.7, Reu = 1.2 3 10, b = 0.9); and a successive-pressure gradient (M = 2.5, Reu = 1.2 3 10, b = 21.0, following a region of b = 0.9). For the favorable-pressure gradient, the turbulent shearstress levels across the boundary layer decreased by 70 – 100%, as compared to the zero-pressure-gradient boundary layer. For the adverse-pressure gradient, a 70 – 100% increase was observed. For the combinedpressure gradient, the shear stresses returned to values similar to the zero-pressure-gradient  ow. A new pressure gradient parameter was found to correlate well with the peak shear-stress ampliŽ cation. It was also postulated that the shear-stress ampliŽ cations were in part the result of the nonuniform bulk dilatation/compression and streamline divergence/convergence, implying a forcing phenomena that in uenced the statistical correlation. The combined-pressure-gradient  ow demonstrated that the turu9v9 bulent structure adjusts relatively rapidly to the distortion. Numerical simulations of the mean velocity obtained with a k-v turbulence model were found to agree very well with the present data. With the exception of the zero-pressure-gradient  ow, the magnitudes of the turbulent shear stresses were not accurately reproduced; however, correct trends were predicted.

Estimation of turbulent shear stress in free jets: application to valvular regurgitation.

In an attempt to better assess the severity of valvular regurgitation, an in-vitro experiment has been conducted to estimate turbulent shear stress levels within free jets issuing from different orifice shapes and sizes by means of hot-wire anemometry. On the basis of the measured mean velocities and the jet profiles, the distributions of the normalized kinematic turbulent shear stress (uv/Um2) were estimated for different jets by using an equation available for self-preserving circular jet. The results indicate that the equation can estimate the distributions of uv/Um2 independent of the orifice shape and Reynolds number of the jet. For the range of Reynolds numbers considered, the estimation of maximum turbulent shear stress inferred from these distributions suggests that the critical shear stress level of approximately 200 N/m2, corresponding to destruction of blood cells, is exceeded for typical blood flow velocity of 5 m/s at the valvular lesion.

Effect of Tilting Disk, Heart Valve Orientation on Flow Through a Curved Aortic Model

The influence of tilting disk valve orientation on pulsatile flow through a curved tube model of the human aorta was studied. Simultaneous, two-component laser Doppler velocimeter measurements were made in a tube having a 22 mm diameter and 41 mm radius of curvature which simulated the average dimensions of the adult aorta. The blood analog fluid had a viscosity of 3.0 cp and matched the refractive index of the glass model aorta. Results at mid-arch showed low turbulence levels in early systole and no influence of valve orientation. During mid-systole, fluid from the ventricle reached mid-arch exhibiting strong influence of valve orientation and increased turbulence levels. With the major orifice of the valve adjacent to the inner curved wall, the peak turbulent shear stress was 307 dynes/cm2 at mid-arch during mid-systole. When the major orifice was rotated 180 degrees, the peak value was reduced to 91 dynes/cm2 at the same location and time. At the exit of the curved section, the flow was independent of the valve orientation and the turbulent shear the flow was independent of the value orientation and the turbulent shear stress levels were an order of magnitude lower than the peak value at the inlet. This study demonstrated that orienting the major orifice of a tilting disk valve adjacent to the outer curved wall minimized turbulent shear stress levels.

Turbulent shear stress measurements in the vicinity of aortic heart valve prostheses

A two dimensional laser Doppler anemometer system has been used to measure the turbulent shear fields in the immediate downstream vicinity of a variety of mechanical and bioprosthetic aortic heart valves. The measurements revealed that all the mechanical valves studied, created regions of elevated levels of turbulent shear stress during the major portion of systole. The tissue bioprostheses also created elevated levels of turbulence, but they were confined to narrow regions in the bulk of the flow field. The newer generation of bioprostheses create turbulent shear stresses which are considerably lower than those created by the older generation tissue valve designs. All the aortic valves studied (mechanical and tissue) create turbulent shear stress levels which are capable of causing sub-lethal and/or lethal damage to blood elements.