scholarly journals Changes in turbulent dissipation in a channel flow with oscillating walls

2012 ◽  
Vol 700 ◽  
pp. 77-104 ◽  
Author(s):  
Pierre Ricco ◽  
Claudio Ottonelli ◽  
Yosuke Hasegawa ◽  
Maurizio Quadrio
Keyword(s):  
Drag Reduction ◽  
Mass Flow Rate ◽  
Channel Flow ◽  
Mass Flow ◽  
Flow Rate ◽  
Plane Channel ◽  
Quasi Equilibrium ◽  
Mean Flow ◽  
Dominant Term ◽  
Turbulent Dissipation

AbstractHarmonic oscillations of the walls of a turbulent plane channel flow are studied by direct numerical simulations to improve our understanding of the physical mechanism for skin-friction drag reduction. The simulations are carried out at constant pressure gradient in order to define an unambiguous inner scaling: in this case, drag reduction manifests itself as an increase of mass flow rate. Energy and enstrophy balances, carried out to emphasize the role of the oscillating spanwise shear layer, show that the viscous dissipation of the mean flow and of the turbulent fluctuations increase with the mass flow rate, and the relative importance of the latter decreases. We then focus on the turbulent enstrophy: through an analysis of the temporal evolution from the beginning of the wall motion, the dominant, oscillation-related term in the turbulent enstrophy is shown to cause the turbulent dissipation to be enhanced in absolute terms, before the slow drift towards the new quasi-equilibrium condition. This mechanism is found to be responsible for the increase in mass flow rate. We finally show that the time-average volume integral of the dominant term is linearly related to the drag reduction.

scholarly journals Detonation initiation by shock focusing at elevated pressure conditions in a pulse detonation combustor

2020 ◽  
Vol 12 ◽  
pp. 175682772092171
Author(s):  
Fabian E Habicht ◽  
Fatma C Yücel ◽  
Joshua AT Gray ◽  
Christian O Paschereit
Keyword(s):  
Success Rate ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Initial Pressure ◽  
Detonation Initiation ◽  
Major Influence ◽  
Mean Flow ◽  
Shock Focusing ◽  
Pulse Detonation

This work contains experimental investigations on the correlation of the detonation initiation process via a shock-focusing device with various initial pressures and mass flow rates. A pulse detonation combustor is operated with stoichiometric hydrogen--air--oxygen mixtures in single cycle operation. A rotationally symmetric shock-focusing geometry evokes the onset of a detonation by the focusing of the reflected leading shock wave, while a blockage plate at the rear end of the test rig is applied to induce an elevated initial pressure. The results show that the reactivity has a major influence on the success rate of detonation initiation. However, measurements with different blockage plates suggest that the mass flow rate has to be considered as well when predicting the success rate. Three main statements can be drawn from the results. (1) An increase in the mean flow velocity induces higher velocity fluctuations which result in a stronger leading shock ahead of the accelerating deflagration front. (2) An increase in the initial static pressure reduces the critical shock strength that must be exceeded to ensure successful detonation initiation by shock focusing. (3) Since the initial pressure is directly linked to the mass flow rate, these contrary trends can cancel each other out, which could be observed for 40% vol. of oxygen in the oxidizer. High-speed images were taken, which confirm that the detonation is initiated in the center of the converging--diverging nozzle due to focusing of the leading shock.

Dynamic Response Formulation for Pneumatically Driven Liquid in a Pressure Vessel

1991 ◽  
Author(s):  
Richard B. Loucks
Keyword(s):  
Mass Flow Rate ◽  
Aluminum Powder ◽  
Mass Flow ◽  
Flow Rate ◽  
Pressure Vessel ◽  
Ideal Gas ◽  
Liquid Oxygen ◽  
Mean Flow ◽  
Containment Vessel ◽  
Thermal Output

Abstract The Thermal Radiation Simulator (TRS) at the U.S. Army Ballistic Research Laboratory uses aluminum powder reacting with liquid oxygen to create a large jet like flame. The flame acts as a large thermally radiant wall, exposing targets to a nuclear weapon equivalent. The aluminum powder is driven pneumatically to the combustion chamber from a pressurized containment vessel. Unfortunately the thermal output of the flame oscillates with large amplitude relative to the mean yield. The fluctuating mass flow rate of aluminum powder from the aluminum powder containment vessel seemed the cause of the unstable output. A computer model of the aluminum vessel was constructed to determine the pressure dynamics in the pressure vessel. The aluminum powder was assumed to behave as a Newtonian liquid. The pneumatic fluid was assumed to be an ideal gas. The model concentrated inside the vessel and at the exit. The result was to determine the mass flow rate of aluminum from the exit given the inlet gas pressures. The model did reveal the source of mass flow fluctuations not to be caused directly by the existing pneumatic set-up. The variation was shown to be perturbated by forces outside the pressure vessel. Once the outside influence was eliminated, the model showed a clean mean flow rate of aluminum powder. The results were applied to the TRS and the thermal output was stabilized.

scholarly journals Money versus time: evaluation of flow control in terms of energy consumption and convenience

2012 ◽  
Vol 700 ◽  
pp. 406-418 ◽  
Author(s):  
Bettina Frohnapfel ◽  
Yosuke Hasegawa ◽  
Maurizio Quadrio
Keyword(s):  
Energy Consumption ◽  
Drag Reduction ◽  
Flow Control ◽  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Transport Sector ◽  
Pumping Power ◽  
Total Energy Consumption ◽  
Practical Applications

AbstractFlow control with the goal of reducing the skin-friction drag on the fluid–solid interface is an active fundamental research area, motivated by its potential for significant energy savings and reduced emissions in the transport sector. Customarily, the performance of drag reduction techniques in internal flows is evaluated under two alternative flow conditions, i.e. at constant mass flow rate or constant pressure gradient. Successful control leads to reduction of drag and pumping power within the former approach, whereas the latter leads to an increase of the mass flow rate and pumping power. In practical applications, however, money and time define the flow control challenge: a compromise between the energy expenditure (money) and the corresponding convenience (flow rate) achieved with that amount of energy has to be reached so as to accomplish a goal which in general depends on the specific application. Based on this idea, we derive two dimensionless parameters which quantify the total energy consumption and the required time (convenience) for transporting a given volume of fluid through a given duct. Performances of existing drag-reduction strategies as well as the influence of wall roughness are re-evaluated within the present framework; how to achieve the (application-dependent) optimum balance between energy consumption and convenience is addressed. It is also shown that these considerations can be extended to external flows.

scholarly journals Change in drag, apparent slip and optimum air layer thickness for laminar flow over an idealised superhydrophobic surface

2013 ◽  
Vol 727 ◽  
pp. 488-508 ◽  
Author(s):  
A. Busse ◽  
N. D. Sandham ◽  
G. McHale ◽  
M. I. Newton
Keyword(s):  
Drag Reduction ◽  
Mass Flow Rate ◽  
Layer Thickness ◽  
Mass Flow ◽  
Flow Rate ◽  
Superhydrophobic Surface ◽  
Slip Length ◽  
Apparent Slip ◽  
Viscosity Contrast ◽  
Gas Layer

AbstractAnalytic results are derived for the apparent slip length, the change in drag and the optimum air layer thickness of laminar channel and pipe flow over an idealised superhydrophobic surface, i.e. a gas layer of constant thickness retained on a wall. For a simple Couette flow the gas layer always has a drag reducing effect, and the apparent slip length is positive, assuming that there is a favourable viscosity contrast between liquid and gas. In pressure-driven pipe and channel flow blockage limits the drag reduction caused by the lubricating effects of the gas layer; thus an optimum gas layer thickness can be derived. The values for the change in drag and the apparent slip length are strongly affected by the assumptions made for the flow in the gas phase. The standard assumptions of a constant shear rate in the gas layer or an equal pressure gradient in the gas layer and liquid layer give considerably higher values for the drag reduction and the apparent slip length than an alternative assumption of a vanishing mass flow rate in the gas layer. Similarly, a minimum viscosity contrast of four must be exceeded to achieve drag reduction under the zero mass flow rate assumption whereas the drag can be reduced for a viscosity contrast greater than unity under the conventional assumptions. Thus, traditional formulae from lubrication theory lead to an overestimation of the optimum slip length and drag reduction when applied to superhydrophobic surfaces, where the gas is trapped.

The standard unit mass flow rate of gas-liquid mixtures

2020 ◽  
pp. 13-16
Author(s):  
V.N. Petrov ◽  
◽  
V.F. Sopin ◽  
L.A. Akhmetzyanova ◽  
Ya.S. Petrova ◽  
...  
Keyword(s):  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate ◽  
Liquid Mixtures ◽  
Unit Mass ◽  
Standard Unit

Prediction of Leakage Mass Flow Rate Through Gas Springs

1987 ◽  
Author(s):  
Roberto Bruno Bossio ◽  
Vincenzo Naso ◽  
Marian Cichy ◽  
Boleslaw Pleszewski
Keyword(s):  
Mass Flow Rate ◽  
Mass Flow ◽  
Flow Rate
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