Klaus Bremhorst

Large-eddy simulation of heat transfer downstream of a backward-facing step

Large-eddy simulation is used to predict heat transfer in the separated and reattached flow regions downstream of a backward-facing step. Simulations were carried out at a Reynolds number of 28 000 (based on the step height and the upstream centreline velocity) with a channel expansion ratio of 1.25. The Prandtl number was 0.71. Two subgrid-scale models were tested, namely the dynamic eddy-viscosity, eddy-diffusivity model and the dynamic mixed model. Both models showed good overall agreement with available experimental data. The simulations indicated that the peak in heat-transfer coefficient occurs slightly upstream of the mean reattachment location, in agreement with experimental data. The results of these simulations have been analysed to discover the mechanisms that cause this phenomenon. The peak in heat-transfer coefficient shows a direct correlation with the peak in wall shear-stress fluctuations. It is conjectured that the peak in these fluctuations is caused by an impingement mechanism, in which...

Spectral measurements of turbulent momentum transfer in fully developed pipe flow

Measurements of the spectral components of turbulent momentum transfer for fully developed pipe flow are presented. The results indicate that near the wall ( y + y + shows that the significant features of the turbulence spectra scale on frequency at any given Reynolds number, thus leading to an interpretation of the flow structure which is consistent with the hydrogen-bubble visualization data of Runstadler, Kline & Reynolds (1963). The results are consistent with a flow model in which disturbances extend from the sublayer to the core of the flow. Recent turbulent heat transfer measurements are also interpreted successfully by this model.

Direct numerical simulation of turbulent Taylor-Couette flow

Direct numerical simulation (DNS) is used to investigate turbulent Taylor-Couette (TC) flow. A simulation was run for a Reynolds number of 3200 in an apparatus with a radius ratio of eta = 0.617 and an aspect ratio of 4.58, which assumed a vortex pair wavelength of 2.29. Results reported include the mean velocity, velocity fluctuation intensities, Reynolds stress budgets, and visualizations of the instantaneous velocity fluctuation field. Secondary near-wall vortex pairs are observed near to the cylinder in addition to the Taylor vortex (TV) motion. Weaker evidence of secondary vortices is found at the outer cylinder where a banded structure has been identified. The azimuthal wall shear stress component shows large peaks and valleys at stagnation points on the surface of both cylinders where flow from neighbouring vortices impacts on the respective wall. These stagnation points correspond to locations where the secondary vortices have been identified. The effect of the mean TV motion is reflected in the Reynolds stress budgets which are similar to but more complex than those of two-dimensional boundary layers. Visualization of the turbulent velocity fluctuations reveals near-wall streaks at the inner cylinder.

A fully compensated hot/cold wire anemometer system for unsteady flow velocity and temperature measurements

Constant-current anemometers operating as resistance thermometers are ideally suited for measurement of stream temperature fluctuations. In order to extend their limited frequency response due to the thermal inertia of the wire, open-loop compensation using a zero-pole network is often used. This method becomes unsatisfactory for unsteady flows with large velocity changes as the time constant of the wire changes significantly from instant to instant. An instantaneous velocity-dependent compensation method is described for use with a velocity-temperature probe. Once set, the compensator time constant matches the wire time constant at all times. Limits for good compensation are presented.

Simulation of cavitation bubbles in a convergent–divergent nozzle water jet

A model for simulating the process of growth, collapse and rebound of a cavitation bubble travelling along the flow through a convergent-divergent nozzle producing a cavitating water jet is established. The model is based on the Rayleigh-Plesset bubble dynamics equation using as inputs ambient pressure and velocity profiles calculated with the aid of computational fluid dynamics (CFD) flow modelling. A variable time-step technique is applied to solve the highly nonlinear second-order differential equation. This technique successfully solves the Rayleigh-Plesset equation for wide ranges of pressure variation and bubble original size and saves considerable computing time. Inputs for this model are the pressure and velocity data from CFD calculation. To simulate accurately the process of bubble growth, collapse and rebound, a heat transfer model, which includes the effects of conduction plus radiation, is developed to describe the thermodynamics of the incondensable gas inside the bubble. This heat transfer model matches previously published experimental data well. Assuming that single bubble behaviour also applies to bubble clouds, the calculated distance from the nozzle exit travelled by the bubble to the point where the bubble size becomes invisible is taken to be equal to the bubble cloud length observed. The predictions are compared with experiments carried out in a cavitation cell and show good agreement for different nozzles operating at different pressure conditions.

Testing for Erosion-Corrosion Under Disturbed Flow Conditions Using a Rotating Cylinder with a Stepped Surface

Abstract Erosion-corrosion is most severe in the vicinity of flow disturbances. In the past, erosion-corrosion under disturbed flow conditions has been studied experimentally in flow loops and nume...

Velocity Bias Associated With Laser Doppler Anemometer Controlled Processors

The controlled processor has been proposed as a means of avoiding velocity bias in laser Doppier anemometry. A theoretical model is presented to show that results free of bias can be obtained if both the ratio of integral time scale to measurement time scale (integral scale data density) and the ratio of sampling time to the measurement time scale (normalized sample interval) are greater than five. Further, by separation of the integral scale data density and normalized sample interval parameters, it is shown that at any integral scale data density the controlled processor will not produce any less bias than a sample and hold processor and no more bias than the free running (unweighted) processor. In some situations it may be considered superior to the sample and hold processor as the data processing and hardware requirements are reduced without any loss of measurement accuracy. Experimental data confirming the theoretical results are also shown. Some of these data are contrary to at least one model available in the literature.

Direct numerical simulation of turbulent flow around a rotating circular cylinder

Turbulent flow around a rotating circular cylinder has numerous applications including wall shear stress and mass-transfer measurement related to the corrosion studies. It is also of interest in the context of flow over convex surfaces where standard turbulence models perform poorly. The main purpose of this paper is to elucidate the basic turbulence mechanism around a rotating cylinder at low Reynolds numbers to provide a better understanding of flow fundamentals. Direct numerical simulation (DNS) has been performed in a reference frame rotating at constant angular velocity with the cylinder. The governing equations are discretized by using a finite-volume method. As for fully developed channel, pipe, and boundary layer flows, a laminar sublayer, buffer layer, and logarithmic outer region were observed. The level of mean velocity is lower in the buffer and outer regions but the logarithmic region still has a slope equal to the inverse of the von Karman constant. Instantaneous flow visualization revealed that the turbulence length scale typically decreases as the Reynolds number increases. Wavelet analysis provided some insight into the dependence of structural characteristics on wave number. The budget of the turbulent kinetic energy was computed and found to be similar to that in plane channel flow as well as in pipe and zero pressure gradient boundary layer flows. Coriolis effects show as an equivalent production for the azimuthal and radial velocity fluctuations leading to their ratio being lowered relative to similar nonrotating boundary layer flows.

Investigation of the flow field of a highly heated jet of air

Measurements of mean and fluctuating velocity and temperature and their self- and cross-products to the third-order are presented for a heated axisymmetric air jet. Froude numbers in the range of 3500 13,190, Reynolds numbers in the range of 3470-8500 and non-dimensional streamwise distances. X*, from 0.27 to 1.98 are covered by the data. It was found that turbulence intensity decreases for the heated jet in the region between the inertia dominated and the buoyancy dominated regions which is contrary to findings with helium jets mixing with ambient air to produce density fluctuations. The effects of heating on the turbulent kinetic energy budget and the temperature variance budget show small differences for the inertia dominated region and the intermediate region which help to explain the transition process to the far field plume region. Constants are evaluated for the isotropic eddy diffusivity and generalised gradient hypothesis models as well as the scalar variance model. No significant effect of heating on the dissipation time-scale ratio was found. A novel wire array with an inclined cold wire was used. Measurements obtained with this probe are found to lead to asymmetries in some of the higher-order products. Further investigation suggested that the asymmetries are attributable to an as yet unreported interference effect produced by the leading prong of the inclined temperature wire, The effect may also have implications for inclined velocity wires which contain a temperature component when used in heated flows

Application of the k-ε turbulence model to the simulation of a fully pulsed free air jet

The work describes application of the [kappa]-[epsilon] turbulence model to a fully pulsed air jet. The standard model failed to predict the change in slope of the velocity decay where the jet changes from pulsed to steady jet behavior. A change in one of the constants of the [kappa]-[epsilon] model based on the behavior of the periodic velocity component relative to the intrinsic component yielded satisfactory results. Features of the pulsed jet which were successfully simulated included the flow reversal near the edge of the jet, increased entrainment when compared to steady jets and large radial outflow near the leading edge of the pulse and large radial inflow near the outer edge of the jet for the remainder of the pulse.