Blocking, gap flow and mountain waves along the coastal escarpment of South Africa

<p>A fatal crash of a light aircraft occurred in the complex coastal mountainous terrain along the South African South Cape in December 2015. An investigation of the meteorological conditions on that day revealed the interaction between mountain waves, gap flow and blocking near a cold front and terrain. The crash made it clear that it is necessary to provide forecasters with knowledge of the turbulence that will arise under these circumstances. Against this background, experiments were carried out near the crash site, with automatic weather stations and radio stations to answer this question. Turbulence has been successfully characterized by the Froude number, Froude altitude scale and thermal wind equation. The Bernoulli equation, which classifies the gap flow, was not helpful due to the effect of the upwind blocking area. Phenomena in descending order of the generated wind force were, compression effect above the peak (44.7 ms<sup>-1</sup>), blocking (26 ms<sup>-1</sup>) and finally gap flow. The gap flow had a negative impact on the strength of the barrier jet. Phenomena in descending order of the turbulence intensity were; gap flow, mountain wave/rotors and finally blocking. Gap flow generated higher vertical speeds than mountain waves. These mountain waves generated the highest vertical speeds measured in South Africa to date, combined with the waves of the shortest wavelength. A blocking jet with a depth of 600 m and a width of 80 km changed the formation of mountain waves significantly. The blocking jet was so strong, that it extended up to 30 km beyond the end of the mountain range. Most likely, a combination of mountain waves, gap flow and blocking contributed to the crash, which shows that these three features cannot be seen as separate processes.</p>

The Quinault Blowdown: A Microscale Wind Event Driven by a Mountain-Wave Rotor

On 27 January 2018, a highly localized, strong wind event occurred along the north shore of Lake Quinault, Washington. The resulting loss of large old-growth trees in a roughly 0.5-km2 region led to blocked roads and power outages. Nearby surface stations did not record anomalous winds, and no tree damage was reported in the surrounding region. Based on public accounts and a nearby seismometer, it appears that the strong winds lasted less than 10 min. Surface and aerial damage surveys showed that the trees fell from a different direction (northerly) than the synoptic or mesoscale f low (southwesterly to southeasterly). Based on high-resolution Weather Research and Forecasting (WRF) Model simulations, it appears that the damaging northerly winds were the result of a strong atmospheric rotor produced by a high-amplitude mountain wave. A simulation with 148-m grid spacing produced a rotor at the same time and location as the treefalls. Synoptic analysis and the high-resolution simulation showed that moderately strong southeasterly flow and a stable layer associated with the approaching occluded front interacted with a ∼750-m-high upstream mountain ridge to produce the mountain wave and associated rotor circulation. The combination of an inversion and strong shear at and above the upstream ridge were outliers in a climatology of soundings from the nearby Quillayute rawinsonde site, suggesting that such intense mountain-wave rotors are unusual in this valley.

Nonsteady Shock Wave Attenuation Due to Leakage

Leakage flow between the rotor and the stator can cause serious performance degradation of wave rotors which utilize nonsteady shock waves to directly transfer energy from burned gases to precompressed air. To solve this problem, primary flow features relevant to leakage are extracted and it was found that the leakage-attributed performance degradation could be abstracted to a special initial-boundary value problem of one-dimensional Euler equations. Then, a general loss assessment method is proposed to solve the problem of nonsteady flow loss prediction. Using the above method, a reasonable physical hypothesis of the initial-boundary value problem depicting the nonsteady leakage flow process is proposed and further, a closed-form leakage loss analytical model combined with an empirical correction method for the discharge coefficient is established. Finally, with the experimentally verified CFD method, comprehensive numerical verification is conducted for the loss prediction model; it is proved that the physical hypothesis of the proposed model in this paper is reasonable and the model is capable of predicting nonsteady shock wave attenuation due to leakage exactly within the range of parameter variations of wave rotors.

Performance Enhancement of One and Two-Shaft Industrial Turboshaft Engines Topped With Wave Rotors

Wave rotors are rotating equipment designed to exchange energy between high and low enthalpy fluids by means of unsteady pressure waves. In turbomachinery, they can be used as topping devices to gas turbines aiming to improve performance. The integration of a wave rotor into a ground power unit is far more attractive than into an aeronautical application, since it is not accompanied by any inconvenience concerning the over-weight and extra dimensioning. Two are the most common types of ground industrial gas turbines: The one-shaft and the two-shaft engines. Cycle analysis for both types of gas turbine engines topped with a four-port wave rotor is calculated and their performance is compared to the performance of the baseline engine accordingly. It is concluded that important benefits are obtained in terms of specific work and specific fuel consumption, especially compared to baseline engines with low compressor pressure ratio and low turbine inlet temperature.

Numerical Modeling of a Wave Turbine and Estimation of Shaft Work

Wave rotors are periodic-flow devices that provide dynamic pressure exchange and efficient energy transfer through internal pressure waves generated due to fast opening and closing of ports. Wave turbines are wave rotors with curved channels that can produce shaft work through change of angular momentum from inlet to exit. In the present work, conservation equations with averaging in the transverse directions are derived for wave turbines, and quasi-one-dimensional model for axial-channel non-steady flow is extended to account for blade curvature effects. The importance of inlet incidence is explained and the duct angle is optimized to minimize incidence loss for a particular boundary condition. Two different techniques are presented for estimating the work transfer between the gas and rotor due to flow turning, based on conservation of angular momentum and of energy. The use of two different methods to estimate the shaft work provides confidence in reporting of work output and confirms internal consistency of the model while it awaits experimental data for validation. The extended wave turbine model is used to simulate the flow in a three-port wave rotor. The work output is calculated for blades with varying curvature, including the straight axial channel as a reference case. The dimensional shaft work is reported for the idealized situation where all loss-generating mechanisms except flow incidence are absent, thus excluding leakage, heat transfer, friction, port opening time, and windage losses. The model developed in the current work can be used to determine the optimal wave turbine designs for experimental investment.

Defining the Thermodynamic Efficiency in a Wave Rotor

A wave rotor enhances the performance of a gas turbine with its internal compression and expansion, yet the thermodynamic efficiency estimation has been troubling because the efficiency definition is unclear. This paper put forward three new thermodynamic efficiency definitions to overcome the trouble: the adiabatic efficiency, the weighted-pressure mixed efficiency, and the pressure pre-equilibrated efficiency. They were all derived from multistream control volumes. As a consequence, they could correct the efficiency values and make the values for compression and expansion independent. Moreover, the latter two incorporated new models of pre-equilibration inside a control volume, and modified the hypothetical “ideal” thermodynamic processes. Parametric analyses based on practical wave rotor data demonstrated that the trends of those efficiency values reflected the energy losses in wave rotors. Essentially, different thermodynamic efficiency definitions indicated different ideal thermal cycle that an optimal wave rotor can provide for a gas turbine, and they were recommended to application based on that essence.