This thesis concentrates on the development of innovative Passive Flow Control (PFC) devices for the simplified reference model (Ahmed body with 25°, 30°, and 35° slant angles) used to represent bluff body vehicles. This PFC is performed by adding linking tunnels at the rear of the bluff body, blowing the flow from the sidewalls (high-pressure zone) to the wake region (low-pressure zone). The movement of a high-pressure flow to the wake zone reduces the wake size, which reduces the pressure drag. A numerical and experimental analysis of this passive method is carried out. The optimal configuration was reached by numerical analysis (showing a drag reduction of up to 5%) and then, experimental tests were performed in a wind tunnel to check the effect of this type of PFC on an Ahmed body 25°. Study the drag reduction performance of the Rear Linking Tunnels (RLTs) in combination with single dielectric barrier discharge (SDBD) actuators located at different positions on the model’s rear part with a spanwise arrangement is the second stage of this project. This active actuator modifies the laminar-to-turbulent transition leading to reattachments of separated flows on the body. A reduction of actuator efficiency when increasing velocity is the main limitation of this method. The main aim of this part is to find a solution for this problem by employing RLTs. Numerical simulations were performed to determine the best position of the active actuators and investigating the combined effect of these two methods at four different freestream velocities (Re= 0.289×106-0.722×106). The positive effect of this combination is noticeable (a 59% increase in the SDBD actuator’s efficiency for Re= 0.722×106), and the non-linear behavior of this combination leads to a high drag reduction of between 13.29% - 17.96% for different Reynolds numbers. The last stage is focused on the effect of the RLTs in combination with Blowing Active Flow Control (BAFC) based on steady and pulsed blowing jets. In this part of the project same as the first stage, the full-scale model of Ahmed body 25° was considered in experimental tests. All the tests were performed at speeds of 10 to 20 m/s (Re =7×105 - 1.2×106). The effect of this steady and pulsed blowing jet system on the drag force of the model and pressure distribution on the surface of the model was studied both for the single AFC method and combined mode with RLTs. Thirteen different configurations for the position of the jet injections were tested to reach the best configurations. Different jet states from steady blowing jet up to pulsed jet (frequency= 10 – 120 Hz) were considered for each of the configurations. High drag reduction up to 16.5% (48% performance enhancement of the Blowing Active Flow Control) is achieved for the combination of RLTs with BAFC.
La tesi si concentra sullo sviluppo di dispositivi passivi di controllo del flusso innovativi (Passive Flow Control (PFC)) per un modello di veicolo semplificato (Ahmed body con angoli di inclinazione di 25 °, 30 ° e 35 °). Questo PFC si ottiene mediante un tunnel di collegamento nella parte posteriore della vettura, soffiando il flusso dalle pareti laterali (zona di alta pressione) alla regione di scia (zona di bassa pressione). L’inserimento di aria ad alta pressione nella zona di scia riduce la dimensione della scia, il che riduce la resistenza dovuta alla pressione. E’ stata eseguita un'analisi numerica e sperimentale di questo metodo passivo. La configurazione ottimale è ottenuta mediante analisi numerica (che mostra una riduzione della resistenza aerodinamica fino al 5%) e successivamente sono stati effettuati test sperimentali in galleria del vento per verificare l'effetto di questo tipo di PFC su un corpo Ahmed 25°. Sono poi state studiate le prestazioni di riduzione della resistenza aerodinamica dei tunnel di collegamento posteriore (RLTs) in combinazione con attuatori di scarica a barriera dielettrica singola (SDBD) situati in diverse posizioni sulla parte posteriore del modello. Questo attuatore attivo modifica la transizione del flusso da laminare a turbolenta, portando al riattacco del flusso nella parte posteriore del veicolo. La principale limitazione di questo metodo è la riduzione dell'efficienza dell'attuatore quando si aumenta la velocità. Lo scopo principale di questa parte è trovare una soluzione a questo problema impiegando RLTs. Sono state eseguite simulazioni numeriche per determinare la migliore posizione degli attuatori attivi e indagare l'effetto combinato di questi due metodi a quattro diverse velocità di flusso libero (Re = 0,289 × 106-0,722 × 106). L'effetto positivo di questa combinazione è evidente (un aumento del 59% dell'efficienza dell'attuatore SDBD per Re = 0,722 × 106) e il comportamento non lineare di questa combinazione porta a un'elevata riduzione della resistenza tra il 13,29% e il 17,96% per i diversi numeri di Reynolds. L'ultima fase è focalizzata sull'effetto degli RLTs in combinazione con il Blowing Active Flow Control (BAFC) basato su getti di soffiaggio costanti e pulsati. In questa parte del progetto, come la prima fase, il modello in scala reale del corpo di Ahmed 25° è stato considerato nei test sperimentali. Tutti i test sono stati eseguiti a velocità da 10 a 20 m / s (Re = 7 × 105 - 1.2 × 106). L'effetto di questo sistema a getto di soffiaggio costante e pulsato sulla forza di resistenza del modello e sulla distribuzione della pressione sulla superficie del modello è stato studiato sia per il metodo AFC singolo che per la modalità combinata con RLTs. Tredici diverse configurazioni per la posizione delle iniezioni a getto sono state testate per raggiungere la migliore prestazione. Per ogni configurazione sono stati considerati diverse condizioni del getto, dal getto soffiante costante fino al getto pulsato (frequenza = 10 - 120 Hz). Un'elevata riduzione della resistenza aerodinamica fino al 16,5% (miglioramento delle prestazioni del 48% del Blowing Active Flow Control) si ottiene per la combinazione di RLTs con BAFC.
Numerical and experimental study of a passive flow control device and its effect on the drag reduction performance of active flow control
Mohammadikalakoo, Babak
2020/2021
Abstract
This thesis concentrates on the development of innovative Passive Flow Control (PFC) devices for the simplified reference model (Ahmed body with 25°, 30°, and 35° slant angles) used to represent bluff body vehicles. This PFC is performed by adding linking tunnels at the rear of the bluff body, blowing the flow from the sidewalls (high-pressure zone) to the wake region (low-pressure zone). The movement of a high-pressure flow to the wake zone reduces the wake size, which reduces the pressure drag. A numerical and experimental analysis of this passive method is carried out. The optimal configuration was reached by numerical analysis (showing a drag reduction of up to 5%) and then, experimental tests were performed in a wind tunnel to check the effect of this type of PFC on an Ahmed body 25°. Study the drag reduction performance of the Rear Linking Tunnels (RLTs) in combination with single dielectric barrier discharge (SDBD) actuators located at different positions on the model’s rear part with a spanwise arrangement is the second stage of this project. This active actuator modifies the laminar-to-turbulent transition leading to reattachments of separated flows on the body. A reduction of actuator efficiency when increasing velocity is the main limitation of this method. The main aim of this part is to find a solution for this problem by employing RLTs. Numerical simulations were performed to determine the best position of the active actuators and investigating the combined effect of these two methods at four different freestream velocities (Re= 0.289×106-0.722×106). The positive effect of this combination is noticeable (a 59% increase in the SDBD actuator’s efficiency for Re= 0.722×106), and the non-linear behavior of this combination leads to a high drag reduction of between 13.29% - 17.96% for different Reynolds numbers. The last stage is focused on the effect of the RLTs in combination with Blowing Active Flow Control (BAFC) based on steady and pulsed blowing jets. In this part of the project same as the first stage, the full-scale model of Ahmed body 25° was considered in experimental tests. All the tests were performed at speeds of 10 to 20 m/s (Re =7×105 - 1.2×106). The effect of this steady and pulsed blowing jet system on the drag force of the model and pressure distribution on the surface of the model was studied both for the single AFC method and combined mode with RLTs. Thirteen different configurations for the position of the jet injections were tested to reach the best configurations. Different jet states from steady blowing jet up to pulsed jet (frequency= 10 – 120 Hz) were considered for each of the configurations. High drag reduction up to 16.5% (48% performance enhancement of the Blowing Active Flow Control) is achieved for the combination of RLTs with BAFC.| File | Dimensione | Formato | |
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https://hdl.handle.net/10589/175291