M. Kurosaka

Kidney and anti-kidney vortices in crossflow jets

Water tunnel experiments were conducted to examine the effect of hole exit geometry on the near-field characteristics of crossflow jets. Hole shapes investigated were round, elliptical, square, and rectangular, all having the same cross-sectional area. Laser-induced fluorescence (LIF) and particle image velocimetry (PIV) were used. The vorticity around the circumference of the jet was tracked to identify its relative contributions to the nascent streamwise vortices, which evolve eventually into kidney vortices downstream. The distinction between sidewall vorticity and that from the leading and trailing edges, though blurred for a round hole, became clear for a square or a rectangular hole. The choice of non-circular holes also made it possible to reveal the unexpected double-decked structures of streamwise vortices and link them to the vorticity generated along the wall of the hole. The lowermost vortex pair of the double-decked structures, located beneath the jet, is what we call a ‘steady’ vortex pair. This pair is always present and has the same sense of rotation as the kidney vortices. The origin of these lower-deck vortices is the hole sidewall boundary layer: as the jet emanates from the hole, the crossflow forces the sidewall boundary layer to roll up into nascent kidney vortices. Here, hole width sets the lateral separation of these steady sidewall vortices. The vortices comprising the upper deck ride intermittently over the top of the ‘steady’ lower pair. The sense of rotation of these upper-deck vortices depends on hole geometry and can be the same as, or opposite to, the lower pair. The origin of the upper deck is the hole leading-edge boundary layer. This vorticity, initially aligned transverse to the crossflow direction, is realigned by the entrainment of crossflow momentum and thus induces a streamwise component of vorticity. Depending on hole geometry, this induced streamwise vorticity can be opposite to the lower-deck vortex pair. The opposing pair, called the ‘anti-kidney’ pair, competes with the nascent kidney-vortices and affects the jet lift-off. The hole trailing-edge boundary layer can likewise be turned toward the streamwise direction. In this case, the turning is caused by the strong reverse flow just downstream of the jet. In the present range of parameters, all hole boundary layer vorticity, regardless of its origin along the hole circumference, is found to influence the kidney vortices downstream.

The influence of vortical structures on the thermal fields of jets

In this investigation we explore the effect of unsteady vortical structures on the adiabatic wall temperature distribution in an impinging jet. Treating first the simpler case of a free jet, we introduce a conceptual model for the separation of the total temperature, appealing to the dynamics of particle pathlines and vortex rings in the jet. The presence of a region of higher total temperature on the inside of the jet and a region of lower total temperature toward the jet periphery, predicted by the model, exhibits good agreement with the experimental data taken at high subsonic Mach number. The results from a numerical simulation further confirm the theoretical expectations.Through a similar argument, we show that when a thermally insulated flat plate is inserted into the jet, the wall temperature distribution is modified by the presence of secondary vortical structures, which are induced near, and swept over, the plate surface. When the plate is near the jet nozzle, a region of lower wall temperature, attributable to these additional vortices, is observed in the experimental data. When the plate is further from the nozzle, no secondary vortices are formed and no region of lowered wall temperature is measured. Self-sustaining acoustic resonance, when it occurs, is found to alter significantly this picture of the wall temperature distribution.Although the scope of this work is limited to free and impinging jets, this present topic, along with the previously reported mechanism of the Eckert–Weise effect, exemplifies the wider family of problems in which unsteady vortical structure strongly affects the wall temperature and heat transfer.

A note on multiple pure tone noise

Abstract A theoretical investigation of multiple pure tone noise is presented. An analysis based on a two-dimensional inviscid flow model is developed to predict the generation and subsequent evolution of multiple pure tone noise from prescribed blade-to-blade non-uniformities in the rotor geometry. The results show that even small non-uniformities within manufacturing tolerances can be a significant source of multiple pure tone noise. Among the non-uniformities investigated, errors in blade spacing are less significant multiple pure tone noise sources than errors in blade stagger or blade contours.

Supersonic cooling by shock–vortex interaction

The subject of total temperature separation in jets was treated in Fox et al. (1993) for subsonic jets. When we extended this study to the case of supersonic jets, we found the presence of a different mechanism of cooling, an effect which does not appear to have been known in the past. Named the ‘shock-induced total temperature separation’, this cooling can be of much greater magnitude than the subsonic cooling treated previously; it is caused by the interaction of convected vortical structures near the jet exhaust with the shock structure of the supersonic jet. In studying this phenomenon, we focus our attention on overexpanded jets exiting a convergent-divergent nozzle. The theoretical results for the shock-induced cooling which are based on a linearized, unsteady supersonic analysis are shown to agree favourably with experiments. When an impingement plate is inserted, the shock-induced cooling would manifest itself as wall cooling, whose magnitude is significantly larger than the subsonic counterpart. This has implications for heat transfer not only in jets, but wherever vortical structures may interact with shock waves.

Improved jet coverage through vortex cancellation

In film cooling used in gas turbines, coolant from compressors is introduced to the hot gas stream of the turbine as crossflow jets. The interaction of the coolant jet and crossflow results in the formation of a pair of counter-rotating vortices, or kidney vortices. The sense of rotation of the kidney vortices is such that they exert two indesirable effects : 1) hot air is forced down beneath the jet to the turbine wall, and 2) the vortices tend to lift the jet off the surface by the mutual induction between the vortex pair. We report the potential of promoting jet attachment by weakening the kidney vortices through cancellation.

The effect of hole geometry on lift-off behavior of coolant jets

Water tunnel experiments have been conducted to understand the effect that hole exit geometry has on the lift-off characteristics of a jet in a crossflow stream. The following basic hole shapes were investigated, all having approximately the same cross-sectional area: round, elliptical, square, and rectangular. Laser Induced Fluorescence (LIF) was used to visualize the jet structure in the region near the hole exit. Both jet trajectory and jet cross-section views provide insight into the basic characteristics of the jet, which are strongly dependent on the formation of kidney-shaped vortices. Particle Image Velocimetry (PIV) was used to obtain the velocity field in the region immediately downstream of the jet. Based on the LIF and PIV results, hole geometry alone was found to influence the basic character of the jet, therefore, effecting the lift-off behavior.

'Coriolis resonance' within a rotating duct

An investigation of the unsteady disturbances of a fixed frequency within a radial duct rotating at a set speed is presented. The flow is assumed to be compressible, inviscid, and of a fluid which is a perfect gas. Equations are developed for the steady and the unsteady parts of the flow in cylindrical coordinates. The unsteady disturbances are expressed by Fourier decomposition in angular position, distance into the duct, and in time. It is found that a resonance is possible when the frequency of flow disturbances is twice the shaft-rotation frequency, considering only the radial and tangential disturbances and not the radial and circumferential disturbances. The particular point at which the resonance occurs indicates the occurrence is due to the Coriolis force, which is only present in the radial and tangential directions. It is noted that the Coriolis force can only be present in open-ended ducts, such as those found in centrifugal compressors.

Theoretical and Experimental Consideration of the Continuous Rotating Detonation Engine

This is a companion paper to ‘Design and experiments of a continuous rotating detonation engine: a spinning wave generator and modulated fuel/ oxidizer mixing’ submitted separately to this conference, which, called Part 1 here, focuses on the design and experiments of the CRDE engine at the University of Washington. A narrow objective of this Part 2 is to develop theories, based on the first-principles, first-order model, for the interpretation of the experimental data of Part 1. But our broader intent is more than that and to extend the conceptual framework wide enough to ferret out the general advantages of the CRDE for air-breathing propulsion and to pinpoint the future research directions. Specifically, we discuss four phenomenologically unique topics: (1) zero total angular momentum in the lab coordinates and its implications, (2) thrust generation by pressure waves trailing detonation waves, (3) interdependence between the mass flow rate, wave number and axial size of the detonation zone, and (4) supersonic expansion in a constant-area CRDE without a throat. Whenever appropriate, we compare them with related fields: spinning and multi-cellular detonations and gas turbines.

VORTEX DYNAMICS ANALYSIS OF UNSTEADY VORTEX WAKES

An incompressible inviscid unsteady vortex dynamics analysis is presented to investigate the flow details for a class of free and bounded shear flows. Using the unsteady vortex models developed for the starting vortex behind sharp edges and the vortex street wakes, particle dynamics computations have been carried out to determine the pathline and streakline patterns in each of these model flows. Based on these results, the energy equations has been integrated along particle paths. The computations reconfirm that an instantaneous separation of energy occurs in the wake. Another interesting phenomenon, termed the Eckert-Weise effect, where the near-wake region contains essentially a cold flow, has also been computationally re-obtained using the present vortex model. Using a similar model, the wall-bounded vortex street wake problem has also been studied. The computed results highlight the important dynamical effects of convecting unsteady vortex motion close to a wall, which indicates that the inviscid entrainment effects play a vital role on the wall layer fluid eruption.

Design and Experiments of a Continuous Rotating Detonation Engine: a Spinning Wave Generator and Modulated Fuel/Oxidizer Mixing

The continuous rotating detonation engine (CRDE) exploits shock-induced combustion in which reactants are ingested and burned by detonation shock waves spinning in an annulus. The high pressure and temperature behind the shock prompt rapid combustion of the fuel. The advantage of using detonation is a gain in total pressure. While deflagration burning always decreases total pressure, detonation burning increases total pressure. This gain is due to the combined effect of the static pressure rise across the shock and the increased motion of fluid behind it, ‘the blast wind’. The significance of total pressure increase is such that the CRDE itself serves as compressor stages, by converting a part of the chemical energy of the fuel directly to compression work. The CRDE acts a ‘bladeless compressor’, which could potentially reduce parts count of compressors and turbines. It is for this very reason why the CRDE is also called a pressure-gain combustor (PGC). At the more fundamental level, detonation is thermodynamically superior to deflagration because after combustion the entropy rise for a given heat input is lower for detonation than for deflagration. As in any heat engine, lower entropy rise leads to higher thermal efficiency, the intrinsic thermodynamic advantage of the CRDE. The paper starts with a survey of the past and recent progress made by many in developing the CRDE technology. With an aim to contribute to advancing further the CRDE technology, in this paper we present the design and experiments of a 14 cm diameter CRDE built and tested at the University of Washington (UW). The UW CRDE has two unique features: (1) direct detonation initiation by a spinning wave generator and (2) regulation of the mixing zone by modulated mixing of fuel/oxidizer.