informazione tecnica: cosa si intende per Ncrit?

[mini star]

ciao a tutti!
sopo una lunga assenza, mi ripresento su questo meeeeeeeraviglioso forum per porre un quesito:

volendo informarmi su un profilo, presente a questo link, mi si è piazzato davanti agli occhi uno sconosciuto e alquanto misterioso valore, chiamato ncrit.
ora, tra voi eminentissimi maestri, v'è qualcuno che sa cosa sia?

grazie baroniani

[Pampa]

Immagino sia Numero di Reynolds Critico che è il valore di Numero di Reynolds in corrispondenza del quale o, meglio, nell'intorno del quale si assiste al passaggio da flusso laminare a flusso turbolento (e viceversa).

[mini star]

Ma 9 o 5 come numero di Reynolds è un po' pochino... Dovrebbe essere intorno a 5 per10 alla 5, che è quello della lastra piana...

[mini star]

A meno che quello indicato non sia l'esponente del 10 o il fattore moltiplicativo del 10 stesso, e non il Re di per sé

[romoloman]

Basterebbe guardare il sito che ci hai linkato tu...

Airfoil lift and drag polar diagrams

Ncrit value - This is used to model of the turbulence of the fluid or roughness of the airfoil. The Xfoil documentation has a section explaining the method and also the table (reproduced below) of suggested values:-
Situation Ncrit
sailplane 12 to 14
motorglider 11 to 13
clean wind tunnel 10 to 12
average wind tunnel 9
dirty wind tunnel 4 to 8

E anche dalla documentazione di Xfoils:

Transition in an XFOIL solution is triggered by one of two ways:

free transition: e^n criterion is met
forced transition: a trip or the trailing edge is encountered

The e^n method is always active, and free transition can occur upstream of the trip. The e^n method has the user-specified parameter "Ncrit", which is the log of the amplification factor of the most-amplified frequency which triggers transition.
A suitable value of this parameter depends on the ambient disturbance level in which the airfoil operates, and mimics the effect of such disturbances on transition. Below are typical values of Ncrit for various situations.

situation Ncrit
----------------- -----
sailplane 12-14
motorglider 11-13
clean wind tunnel 10-12
average wind tunnel 9 <= standard "e^9 method"
dirty wind tunnel 4-8

Note: The e^n method in XFOIL is actually the simplified envelope version, which is the same as the full e^n method only for flows with constant H(x). If H is not constant, the two methods differ somewhat, but this difference is typically within the uncertainty in choosing Ncrit.

The e^n method is only appropriate for predicting transition in situations where the growth of 2-D Tollmien-Schlichting waves via linear instability is the dominant transition-initiating mechanism.
Fortunately, this happens to be the case in a vast majority of airfoil applications. Other possible mechanisms are:

* Crossflow instabilities. These occur on swept wings with significant favorable chordwise pressure gradients.

* Attachment-line transition. This requires large sweep, large LE radius, and a large Reynolds number. Occurs primarily on big jets.

* Bypass transition. This occurs in cases with sufficient wall roughness and/or large freestream turbulence or vibration levels.
The linear-instability phase predicted by the e^n method is "bypassed", giving relatively early transition. Usually occurs in favorable pressure gradients, while the linear-instability mechanism usually dominates in adverse pressure gradients.

If any of these alternative transition mechanisms are present, the trips must be set to mimick their effect. The bypass transition mechanism can be mimicked to some extent by the e^n method by setting Ncrit to a small value --- Ncrit=1 or less. This will cause transition just after linear instability begins. For very large freestream turbulence or roughness in favorable pressure gradients, bypass transition can occur before the linear instability threshold, and in this case trips will have to be set as well.

[mini star]

Originalmente inviato da romoloman

Basterebbe guardare il sito che ci hai linkato tu...

Airfoil lift and drag polar diagrams

Ncrit value - This is used to model of the turbulence of the fluid or roughness of the airfoil. The Xfoil documentation has a section explaining the method and also the table (reproduced below) of suggested values:-
Situation Ncrit
sailplane 12 to 14
motorglider 11 to 13
clean wind tunnel 10 to 12
average wind tunnel 9
dirty wind tunnel 4 to 8

E anche dalla documentazione di Xfoils:

Transition in an XFOIL solution is triggered by one of two ways:

free transition: e^n criterion is met
forced transition: a trip or the trailing edge is encountered

The e^n method is always active, and free transition can occur upstream of the trip. The e^n method has the user-specified parameter "Ncrit", which is the log of the amplification factor of the most-amplified frequency which triggers transition.
A suitable value of this parameter depends on the ambient disturbance level in which the airfoil operates, and mimics the effect of such disturbances on transition. Below are typical values of Ncrit for various situations.

situation Ncrit
----------------- -----
sailplane 12-14
motorglider 11-13
clean wind tunnel 10-12
average wind tunnel 9 <= standard "e^9 method"
dirty wind tunnel 4-8

Note: The e^n method in XFOIL is actually the simplified envelope version, which is the same as the full e^n method only for flows with constant H(x). If H is not constant, the two methods differ somewhat, but this difference is typically within the uncertainty in choosing Ncrit.

The e^n method is only appropriate for predicting transition in situations where the growth of 2-D Tollmien-Schlichting waves via linear instability is the dominant transition-initiating mechanism.
Fortunately, this happens to be the case in a vast majority of airfoil applications. Other possible mechanisms are:

* Crossflow instabilities. These occur on swept wings with significant favorable chordwise pressure gradients.

* Attachment-line transition. This requires large sweep, large LE radius, and a large Reynolds number. Occurs primarily on big jets.

* Bypass transition. This occurs in cases with sufficient wall roughness and/or large freestream turbulence or vibration levels.
The linear-instability phase predicted by the e^n method is "bypassed", giving relatively early transition. Usually occurs in favorable pressure gradients, while the linear-instability mechanism usually dominates in adverse pressure gradients.

If any of these alternative transition mechanisms are present, the trips must be set to mimick their effect. The bypass transition mechanism can be mimicked to some extent by the e^n method by setting Ncrit to a small value --- Ncrit=1 or less. This will cause transition just after linear instability begins. For very large freestream turbulence or roughness in favorable pressure gradients, bypass transition can occur before the linear instability threshold, and in this case trips will have to be set as well.

Grazie mille per la risposta.... Quindi, se ho capito bene, è un numero che rappresenta i disturbi nelle misurazioni dovuti a caratteristiche del profilo o della galleria del vento?

[Pampa]

Originalmente inviato da Pampa

Immagino sia Numero di Reynolds Critico che è il valore di Numero di Reynolds in corrispondenza del quale o, meglio, nell'intorno del quale si assiste al passaggio da flusso laminare a flusso turbolento (e viceversa).

Ho detto una vaccata, scusate.

[romoloman]

Originalmente inviato da Pampa

Ho detto una vaccata, scusate.

Neanche più di tanto, visto che non è il numero critico di Reynolds ma è un parametro matematico che è abbastanza correlato.
A differenza del numero di Reynolds questo è più una stima delle condizioni al contorno... legato al profilo ma anche all'ambiente di misurazione.
Ovvero come da traduzione è in parametro il cui valore dipende dal livello di disturbo del flusso nell'ambiente in cui il profilo opera e influenza l'effetto di tali disturbi nel punto di transizione da flusso laminare a flusso turbolento.

[mini star]

grazie mille, ho capito

a presto!

Mattia